This invention relates to an X-ray apparatus used in the field of diagnosis and more particularly to an X-ray photographing apparatus using an X-ray image intensifier and correcting the brightness of the output image.
With an X-ray photographing apparatus using an X-ray image intensifier, X-rays emitted from an X-ray source penetrate a subject to the image intensifier to be converted into visible light and to be amplified, thereby providing a light image.
With the known X-ray photographing apparatus 10 arranged as shown in FIG. 1, X-rays emitted from an X-ray source 12 are irradiated on a subject 14 and make an X-ray image of the subject 14 after passing it through. The X-ray image obtained is conducted through a grid 16 eliminating scattering X-rays and then projected to an input screen of an X-ray image intensifier 18 to be converted into a light image and to be amplified and the amplified light image are produced on an output screen. Then the light image is picked up through a lens system 20 and is focussed on a film received in an optical camera 22. An X-ray image of the subject 14 is projected on the film through the above-mentioned process. To provide an optinum X-ray photograph, the light image delivered from the lens system 20 is transferred to a television camera 24 by means of a mirror 23, thereby effecting X-ray photographing while observation is made on a television monitor 26 connected to the television camera 24. Thus, an output light image is obtained in the form of not only a film image by the aid of the optical camera 22, but also an image produced on the television monitor 26. Sometimes, an image produced on the output screen of the intensifier 18 is utilized intact through observation by an operator, instead of being conducted through the television monitor 26. However, for example, a film image obtained by the prior art X-ray photographing apparatus has the drawbacks that little light is projected on the peripheral portion of the film image due to three main causes described below, leading to a decrease in the degree of blackening on the peripheral portion and sometimes in consequence a substantial disappearance of an image from the peripheral portion.
The first cause is that the intensity of X-rays emitted from the X-ray source 12 is not uniformly distributed. As illustrated in FIG. 2a, electrons emitted from a cathode filament 29 received in the X-ray tube 28 of the X-ray source 12 impinge on an anode target 30. X-rays produced on an electron-impact surface of the anode target 30 are picked up through a radiating window 31. Since the electron-impact surface of the anode target 30 is inclined relative to the axis of the X-ray tube 28, X-rays projected on a plane parallel with the axis do not present an axially symmetric intensity distribution relative to a perpendicular from the focus of the anode target 30 to the projection plane. Namely the intensity of X-rays is distributed such that it is maximum in the direction perpendicular to the surface of the anode target and minimum or zero in the direction tangential to the surface thereof. Generally, X-rays have an intensity inversely proportional to a distance through which they travel. Therefore, X-rays passing through by the peripheral portion of the radiating window 31 travel for a longer distance than those conducted through the central portion of the radiating window 31 and in consequence decrease in intensity on the projection plane than the X-rays which are emitted through the central portion. As the result, X-rays vertically thrown on the projection plane present an intensity distribution represented as the solid line, as shown in FIG. 2b, by an asymmetric curve which becomes extremely small on one side of the curve and most prominent in a region defined between the other side and central portion of the curve. While X-rays horizontally thrown on the projection plane present an intensity distribution represented as the dot line.
The second cause leading to the non-uniform brightness of a film image is that X-rays attenuate spatially unevenly in the intensifier 18. Even when X-rays initially having the uniform intensity distribution enter the intensifier 18, an output light image therefrom does not indicate a uniformly distributed brightness. Now let it be assumed that X-rays 33a, 33b having the same intensity are brought into the intensifier 18 from the X-ray source 12. X-rays entering a tube envelope 34 of the intensifier 18 pass to an input screen 36 through a face plate 35. X-rays conducted through an aluminium base plate 36a reach a fluorescent layer 36b to be converted into light therein. X-rays 33b obliquely entering the intensifier 18 travel a longer distance to the face plate 35 than X-rays 33a entering the face plate 35 at right angles and also have to run a longer distance through the face plate 35 than the latter X-rays 33a. Therefore, the X-rays 33b are more attenuated and have a lower intensity than the X-rays 33a. Moreover, the X-rays 33b obliquely entering the fluorescent layer 36b present a longer fluorescent length with the resultant decline in the degree of resolution. Nevertheless, an attempt progressively to reduce the thickness of the fluorescent layer 36b from the center toward the periphery in order to unify the fluorescent lengths produced by X-rays would decrease the X-rays to light conversion efficiency. Further, due to the distortion an output light image appearing on an output screen 38 takes the pincushion distortion whose peripheral portions are larger than the central portion. Therefore, an amount of light per unit area in the peripheral portions of the output light image is smaller than in the central portion thereof, indicating a relatively lower brightness. For the above-described reason, an output light image presented on the output screen 38 of the intensifier 18 has a non-uniform brightness, namely, as shown in FIG. 5b, the brightness reaches a maximum level in the central portion of the output screen 38 and indicates a minimum level represented by an axially symmetric curve in the peripheral portions of the output screen 38. The brightness in the peripheral portions accounts for about 60% of that of the central portion.
The third cause giving rise to the uneven brighness of an output light image projected on a film is that the image focussed by the lens system 20 itself has a non-uniform brightness. As seen in FIG. 4a, the lens system 20 generally comprises a pair of convex lenses 40, 41. The forward lens 40 converts light from the intensifier 18 into parallel light, and the rear lens 41 focus the parallel light. Light starting from the central point 43a of the light image 43 goes straight to the central point 44a of the focussed image 44. Light sent forth from the upper point 43b of the light image 43 is projected on the lower point 44b of the focussed image 44. Light from the central point 43a of the light image 43 is introduced at an angle falling within the range of a solid angle .alpha., whereas light from the upper point 43b of the light image 43 is supplied at an angle falling within the range of a solid angle .beta.. Since the solid angle .beta. is smaller than the solid angle .alpha. and the central line thereof is inclined, light emitted from the upper point 43b of the light image 43 has a smaller amount than light emitted from the central point 43a thereof. When converted into a parallel form by the forward lens 40, light from the upper point 43b is not rendered fully parallel with the optical axis of the forward lens 40. Namely, a bundle of the parallel light from the upper point 43b is slightly inclined as a whole from the optical axis of the forward lens 40. Part of the light is displaced from the rear lens 41 and does not contribute to the convergence of light from the light image 43 at the lower point 44b of the focussed image 44. This event also arises with respect to light emitted from the lower point 43b of the light image 43. As shown in FIG. 4b, therefore, light occupying the peripheral portions of a light image focussed by the lens system 20 has a smaller amount than those of the central portion of the focussed image. As the result, the brightness of the focussed light image has a distribution axially symmetical with respect to the axis of the image.
The above-mentioned three causes collectively give rise to a considerable decrease in an amount of light lying in the peripheral portions of a light image projected on a film received in the optical camera from that of light occupying the central portion of the light image. Where an X-ray tube is set upright, as shown in FIG. 2 and the target 30 is positioned above the cathode filament 29, a circular light image indicated in FIG. 5a has a brightness in the vertical direction which is distributed as shown in FIG. 5b and a brightness in the horizontal direction which is distributed as indicated in FIG. 5c. The brightness of the light image in the vertical direction has a distribution represented by axially asymmetric curve in which a maximum brightness appears in a region defined between one end and the center of the light image. The brightness of the light image in the horizontal direction has a distribution denoted by an axially symmetric curve in which a maximum brightness is presented in the central portion of the light image.
The foregoing description refers to an output light image projected on a film. Neither of an output light image from the intensifier 18 and an output light image from the television monitor 26 fails to have a uniform brightness from the effect of the X-ray source 12 and intensifier 18, to say nothing of the effect of the optical camera.
Various attempts have hitherto been made to eliminate drawbacks accompanying the prior art X-ray photographing apparatus. One of the known X-ray photographing apparatuses is the type in which a glass face plate 35 has its thickness increased in the central portion and progressively reduced toward the periphery to cause X-rays to be attenuated at a uniform rate. With this apparatus, however, it is necessary to emit a larger dosage of X-rays in order to produce a light image having a uniform brightness. Where a subject 14 is a human body, and, for example, his stomach is photographed by X-rays, application of an increased dosage of X-rays very adversely affects the human body, for example, exerts a harmful effect on a gene or gives rise to the more frequent occurrence of leukemia. Further, increased dosage of X-rays results on a larger absorption of X-rays in the central portion of the inlet screen 36, and more prominent scattering of X-rays, thereby causing a light image to present a less distinct contrast of an output image. Another prior art X-ray photographing apparatus is the type wherein a metal back of the output screen of the intensifier 18 which is formed by vacuum deposition of an aluminum layer has its thickness increased at the center, thereby purposely causing the brightness of a light image to be progressively decreased toward the center of the light image. In this case, too, in comparison with the known apparatus having an aluminium layer where thickness is uniform, X-rays must be applied profusely, with the resultant drawback that the central portion of the input screen 36 becomes unduly bright, leading to an increase in an amount of reflected light at the input screen and the adjacent area thereof and in consequence a less distinct contrast in the light image.
With still another conventional X-ray photographing apparatus, a mask bored with a smaller hole than the diameter of a forward lens 40 is provided as an iris diaphragm between the forward and rear lenses 40, 41 in order to prevent an amount of light from being reduced in the periphery of a light image delivered from the lens system 20. This arrangement can indeed provide a focussed image having a uniform brightness by cutting off the peripheral portions of a bundle of parallel light passing through the forward lens 40. But removal of the peripheral portion of the bundle of parallel light decreases an amount of light taking part in providing a focussed image. Therefore, this prior art X-ray photographing apparatus is still accompanied with the drawback that an increased dosage of X-rays has to be applied in order to obtain a fully bright focussed image.